type DB struct {
  // ...
	meta0    *meta
	meta1    *meta  
}

type meta struct {
  // ...
	root     bucket	// 根bucket的信息
}

type Bucket struct {
	*bucket

  // ...
  buckets  map[string]*Bucket // 存储子bucket的对应关系
}

type bucket struct {
	// 根节点的page id
	root pgid // page id of the bucket's root-level page
	// 单调递增的序列号
	sequence uint64 // monotonically incrementing, used by NextSequence()
}

type Bucket struct {
	*bucket
	tx       *Tx                // the associated transaction
	buckets  map[string]*Bucket // subbucket cache
	page     *page              // inline page reference
	rootNode *node              // materialized node for the root page.
	nodes    map[pgid]*node     // node cache

	// Sets the threshold for filling nodes when they split. By default,
	// the bucket will fill to 50% but it can be useful to increase this
	// amount if you know that your write workloads are mostly append-only.
	//
	// This is non-persisted across transactions so it must be set in every Tx.
	FillPercent float64
}

// leafPageElement represents a node on a leaf page.
type leafPageElement struct {
	flags uint32
	pos   uint32
	ksize uint32
	vsize uint32
}

const (
	bucketLeafFlag = 0x01
)

func (b *Bucket) Bucket(name []byte) *Bucket {
	// Move cursor to key.
	// 否则创建一个Cursor查询
	c := b.Cursor()
	k, v, flags := c.seek(name)

	// Return nil if the key doesn't exist or it is not a bucket.
	// 查询不到，或者不是子bucket节点，都返回nil
	if !bytes.Equal(name, k) || (flags&bucketLeafFlag) == 0 {
		return nil
	}

	// Otherwise create a bucket and cache it.
	// 打开这个bucket并且cache到buckets map中
	var child = b.openBucket(v)
	if b.buckets != nil {
		b.buckets[string(name)] = child
	}

	return child
}

func (b *Bucket) openBucket(value []byte) *Bucket {
	// 创建一个bucket
	var child = newBucket(b.tx)

	// If this is a writable transaction then we need to copy the bucket entry.
	// Read-only transactions can point directly at the mmap entry.
	if b.tx.writable {
		child.bucket = &bucket{}
		*child.bucket = *(*bucket)(unsafe.Pointer(&value[0]))
	} else {
		child.bucket = (*bucket)(unsafe.Pointer(&value[0]))
	}

	// Save a reference to the inline page if the bucket is inline.
	// inline bucket
	if child.root == 0 {
		// bucket的page
		child.page = (*page)(unsafe.Pointer(&value[bucketHeaderSize]))
	}

	return &child
}

// inlineable returns true if a bucket is small enough to be written inline
// and if it contains no subbuckets. Otherwise returns false.
// 返回这个bucket是否能够inline操作
func (b *Bucket) inlineable() bool {
	var n = b.rootNode

	// Bucket must only contain a single leaf node.
	// 如果没有根节点，或者根节点不是叶子节点的不能进行inline操作
	if n == nil || !n.isLeaf {
		return false
	}

	// Bucket is not inlineable if it contains subbuckets or if it goes beyond
	// our threshold for inline bucket size.
	// 有子bucket，或者大小超过maxInlineBucketSize阈值的，都不能进行inline操作
	var size = pageHeaderSize
	for _, inode := range n.inodes {
		size += leafPageElementSize + len(inode.key) + len(inode.value)

		if inode.flags&bucketLeafFlag != 0 {
			return false
		} else if size > b.maxInlineBucketSize() {
			return false
		}
	}

	return true
}

// Returns the maximum total size of a bucket to make it a candidate for inlining.
func (b *Bucket) maxInlineBucketSize() int {
	return b.tx.db.pageSize / 4
}

type Cursor struct {
	bucket *Bucket		// 对应的bucket
	stack  []elemRef	// 存储递归遍历时中间过程的栈，由于栈是先进后出结构，所以遍历的过程中层次高的在栈的低端。
}

// 光标移动过程中，中间过程的信息
type elemRef struct {
	page  *page	// 页面
	node  *node	// 内存中的页面信息
	index int		// 保存在当前page、node遍历到了哪个节点
}

// 移动到bucket的第一个元素上，返回其key value数据
func (c *Cursor) First() (key []byte, value []byte) {
	_assert(c.bucket.tx.db != nil, "tx closed")
	// 修改stack只保存第一个stack，及栈底部
	c.stack = c.stack[:0]
	// 返回root节点的page、node
	p, n := c.bucket.pageNode(c.bucket.root)
	// 将root节点所在的page、node信息放到栈顶，index为0，表示从第一个子节点开始遍历
	c.stack = append(c.stack, elemRef{page: p, node: n, index: 0})
	// 移动到当前的第一个元素
	// first函数做的事情：从树顶端开始，从最左边一直往下遍历到叶子节点为止
	// 因为B树是中序遍历的，所以最左边的节点数据最小
	c.first()

	// If we land on an empty page then move to the next value.
	// https://github.com/boltdb/bolt/issues/450
	// 如果没有元素，就移动到下一个元素
	if c.stack[len(c.stack)-1].count() == 0 {
		c.next()
	}

	// 拿到k、v、flags
	k, v, flags := c.keyValue()
	// 如果是子bucket，就返回k以及nil
	if (flags & uint32(bucketLeafFlag)) != 0 {
		return k, nil
	}
	return k, v

}

// first moves the cursor to the first leaf element under the last page in the stack.
// 找到stack最后一个页面中的第一个叶子节点
func (c *Cursor) first() {
	for {	// 找到最左边第一个叶子节点
		// Exit when we hit a leaf page.
		// 每次循环取出最后一个元素
		var ref = &c.stack[len(c.stack)-1]
		if ref.isLeaf() {	// 如果是叶子节点就退出循环，即这个循环终止的条件是向下一直找到叶子节点为止
			break
		}

		// Keep adding pages pointing to the first element to the stack.
		// 根据ref.index拿到pgid
		var pgid pgid
		if ref.node != nil {
			pgid = ref.node.inodes[ref.index].pgid
		} else {
			pgid = ref.page.branchPageElement(uint16(ref.index)).pgid
		}
		// 拿到对应的page、node
		p, n := c.bucket.pageNode(pgid)
		// 放到栈顶，注意这里的index是0
		// 即向下查找的时候取的都是最左边的节点
		c.stack = append(c.stack, elemRef{page: p, node: n, index: 0})
	}
}